Cathode-ray tube

ABSTRACT

In a cathode ray tube, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are focused on the electron-receiving screen of the tube; the focusing lens includes first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, and a third annular electrode assembly extending between the first and second electrodes and which includes an axial array of at least three annular electrode portions, the electrode portions at the ends of the axial array being at a different potential than the first and second electrodes to establish the focusing electric field and at least one of the electrode portions intermediate the end portions being at a potential that deviates from the potential of the end electrode portions in a direction toward the potential of the first and second electrodes to modify the electric field so that its equivalent optical lens has relatively flatter surfaces for further reducing aberrations of the beam or beams focused thereby.

United States Patent Miyaoka [54] CATHODE-RAY TUBE [72] inventor: Senri Miyaoka, Kanagawa-ken, Japan [73] Assignee: Sony Corporation, Tokyo, Japan [22] Filed: Aug. 26, 1969 [21] Appl. No.: 853,169

[ 30] Foreign Application Priority Date Dec. 19, 1968 Japan ..43/93590 [52] [1.8. CI.'...'. ..315/3l TV, 315/16 [51] Int. Cl. ..H0lj 29/56 [58] Field ofSearch ..315/31, 16

[56] References Cited UNITED STATES PATENTS 3,501,673 3/1970 Compton ..3l5/31 2,351,757 6/1944 Gray ..315/31X op-tg i i 7 Attorney-Albert C. Johnston, Robert E. Isner, Lewis H. Eslinger and Alvin Sinderbrand [5 7] ABSTRACT In a cathode ray tube, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are focused on the electron-receiving screen of the tube; the focusing lens includes first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, and a third annular electrode assembly extending between the first and second electrodes and which'includes an axial array of at least three annular electrode portions, the electrode portions at the ends of the axial array being at a different potential than the first and second electrodes to establish the focusing electric field and at least one of the electrode portions intermediate the end portions being at a potential that deviates from the potential of the end electrode portions in a direction toward the potential of the first and second electrodes to modify the electric field so that its equivalent optical lens has relatively flatter surfaces for further reducing aberrations of the beam or beams focused thereby.

7 Claims, 6 Drawing Figures PATENTED MAR 28 I97? 3, 5 52 g 9 5 FIG. 3A. 7

POff/VT/AL FIG. 3B. 2

L11 [III 111/ 111111 [Ill/Ill FIG. 30.

SENRI MIYAOKA ATTORNEY CATll-IOIDIE-RAY TUBE This invention generally relates to cathode ray tubes, and is particularly directed to improvements in the electrostatic lens of such tubes by which the electron beam or beams are focused on the electron-receiving screen ofthe tube.

In cathode ray tubes having an electron gun of the unipotential type, each electron beam is passed through the electric field of the electrostatic focusing lens so as to be focused thereby on the screen, and such lens usually comprises first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, and a third annular electrode extending between the first and second electrodes and being at a different potential, for example, at a substantially lower potential, to establish the focusing electric field. In order to minimize spherical aberrations of the beam focused on the screen, it is desirable that the electrostatic focusing lens be equivalent to an optical lens of large diameter and having relatively flat surfaces, that is, surfaces with large radii of curvature:

In an electrostatic focusing lens, the surface curvatures of the equivalent optical lens are dependent upon the gradient of the potential along the optical axis between the first and second electrodes, and the gradient of the potential is, in turn, dependent upon the potential difference between the first and second electrodes and the third or intermediate electrode and also upon the axial distance between the first and second electrodes and the diameter of the third electrode. Since the electron gun is positioned within the neck of the cathode ray tube envelope, it will be apparent that the diameter of the third or intermediate electrode of the electrostatic focusing lens is limited by the diameter of the neck. Thus, the diameter of the equivalent optical lens can be increased by increasing the diameter of the intermediate electrode only to a limited extent. If the axial distance between the first and second or end electrodes of the electrostatic focusing lens is reduced to decrease the potential gradient in the field along the optical axis, and hence to increase the radii of curvature of the surfaces of the equivalent optical lens, the focusing effect of the lens is decreased and, therefore, it is necessary to undesirably increase the distance from the focusing lens to the screen and also the overall length of the tube. If the potential difference between the intermediately electrode and the end electrodes is to be reduced, it becomes necessary to apply a relatively high voltage to the intermediate electrode, bearing in mind that the end electrodes are usually at the anode voltage which is generally in the range of from 13 to 20 kv. The application of a relatively high voltage, such as, a voltage of4 to 5 kv. or more, to the intermediate electrode is disadvantageous in that it requires additional circuitry for producing that high voltage, and further in in that there is the possibility of discharges occurring between the closely spaced leads and pins that supply the voltages to the intermediate electrode and to the grids by which the beam is produced and modulated.

The need for an electrostatic focusing lens of single-gun, equivalent to an optical lens of large diameter and having surfaces of large radii of curvature is particularly acute in the case of single-gun, plural-beam cathode ray tubes of the type disclosed in US. Pat. No. 3,448,316, issued June 3, 1969, and having a common assignee herewith.

In such single-gun, plural-beam cathode ray tubes adapted for use as a color picture tube in a television receiver, a cathode structure emits electrons which are formed into a plurality of electron beams and such beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens which is common to all the beams and focuses the beams on the electron-receiving screen. The fact that all of the beams pass through the center of the focusing lens diminishes the aberrations introduced by the latter as compared with earlier proposed arrangements in which at least two of the beams pass through the focusing lens at substantial distances from the optical axis. However, optimum reduction of aberration again requires that the electrostatic focusing lens for focusing the beams which converge to cross each other at its optical center be equivalent to an optical lens of large diameter having surfaces of large radii of curvature.

One suitable method of achieving an electrostatic focusing lens equivalent to an optical lens of large diameter and having surfaces of large radii of curvature is disclosed in my copending patent application, Ser. No. 846,533, filed July 21, I969 corresponding to Japanese Pat. application No. 93589/68, filed Dec. 19, 1968; wherein an auxiliarly electrode is disposed within the intermediate electrode of the lens to reduce the potential gradient in the focusing electric field; however, there are other suitable methods of achieving this result without the necessity of utilizing an auxiliary electrode.

Accordingly, it is an object of this invention to provide a unipotential, focusing type electron gun for a single-beam or plural-beam cathode ray tube in which the electrostatic focusing lens is made equivalent to an optical lens of large diameter having surfaces of large radii of curvature, without unduly increasing the diameter of the tube neck or the length of the tube and further without reducing the focusing effect of the lens.

In a cathode ray tube according to an aspect of this invention, for example, a color picture tube in which a plurality of electron beams are made to converge or cross each other substantially at the optical center of an electrostatic focusing lens by which the beams are all focused on the electron-receiving screen of the tube; the electrostatic focusing lens is constituted by first and second axially spaced, annular electrodes extending around the tube axis and being at the same potential, for example, approximately the anode voltage of the tube, and a third annular electrode assembly extending between the first and second electrodes and which includes an axial array of at least three annular electrode portions, the electrode portions and the ends of the axial array being at a different potential than the first and second electrodes, for example, a voltage substantially lower than the anode voltage, to establish the focusing electric field, while at least one of the electrode portions intermediate the end portions is at a potential that deviates from the potential of the end electrode portions, in a direction toward the potential of the first and second electrodes, for example, the same potential of approximately the anode voltage of the tube, so as to reduce the potential gradient in the focusing electric field and thereby make the electrostatic lens equivalent to a large diameter optical lens having surfaces with large radii of curvature.

In an electrostatic focusing lens for a cathode ray tube, as aforesaid, the axial array preferably comprises an odd number of electrode portions spaced apart from each other, with the one electrode portion being in the middle of the array.

The above, and further object, features and advantages of the invention, will appear from the following detailed description of illustrative embodiments of the invention which is to be read in conjunction with the accompanying drawings, in which:

FIG. 1 is a diagrammatic axial sectional view of a conventional single-beam, unipotential focusing electron gun;

FIG. 2 is a fragmentary axial sectional view of an electrostatic focusing lens in accordance with an embodiment of the present invention;

FIG. 3A is a graphical illustration of the variations of potential along the tube axis in focusing lenses according to the prior art and this invention, respectively;

FIGS. 3B and 3C are graphical illustrations of the lines of equal potential in focusing lenses according to the prior art and this invention, respectively;

FIG. 4 is a fragmentary axial sectional view of a color picture tube employing an electrostatic focusing lens according to the present invention.

Referring to the drawings in detail, and initially to FIG. 1 thereof, it will be seen that a conventional single-beam unipotential electron gun 10 for a cathode ray tube is there shown to include a cathode 11 constituting an electron beam generating source, first and second control grids l2 and 13 having aligned apertures 14 and 15, respectively, and an electrostatic focusing lens 16. The lens 16 includes first and second end electrodes 17 and 18 which are annular, axially spaced from each other and coaxial with the tube axis x-x, and a relatively larger diameter third or intermediate annular electrode 19 which is also coaxial with the tube axis and extends between end electrodes 17 and 18 and axially overlaps the latter.

In operating the electron gun It), appropriate voltages are applied to grids 12 and 13 and to electrodes 17, 18 and 19. For example, with the voltage of cathode 11 as a reference, a voltage of() to 400 v. is applied to first grid 12 for modulating the beam, a voltage ofO to 500 v. is applied to grid 13 to cause the bundle of electrons of the single-beam to converge to a point source substantially within aperture 15, a voltage of 13 to 20 kv. is applied to the intermediate electrode 19.

The voltage applied to electrodes 17 and 18 may conveniently be anode voltage applied to the conductive layer 24 at the inner surface of the tube which further has a phosphor screen 23 for receiving the electrons of the single beam 25. With the distribution of applied voltages, as given above, the bundle or rays of electrons of beam 25 which diverge from the tube axis x-x after passing through aperture 15 are converged or focused at a point on screen 23 by passage through the electric field thus established within electrode 19 between electrodes 17 and 18, and which is equivalent to an optical lens represented in broken lines at L on FIG. 1 and centered between electrodes 17 and 18.

When the axial distance between electrodes 17 and 18 is sufficient to ensure that the overall length of the tube is not undersirably large and the diameter of electrode 19 is suitable to permit a reasonable diameter of the tube neck, the field of electrostatic focusing lens 16 has a steep potential gradient as indicated by the curve P on FIG. 3A which represents the potentials along the tube axis x-x at various distances from the plane yy passing through the optical center of lens 16 or its equivalent optical lens L. With such a steep potential gradient, the equivalent optical lens L is of limited diameter and has surfaces with relatively small radii of curvature. As shown graphically on FIG. 3B, the small radii of curvature of the surfaces of equivalent optical lens L result from the steep gradient of the lines p of equal potential within the electric field of electrostatic focusing lens 16, which lines p as shown, are at substantial angles with respect to axis x-x.

Thus, in focusing beam 25, the conventional electrostatic lens 16 may impart spherical aberrations to the beam with resultant poor resolution of the picture produced when the beam is made to scan screen 23, as by the usual deflection yoke (not shown).

In accordance with this invention, the above-mentioned spherical aberrations are substantially diminished by providing an axial array of at least three electrode portions in place of the conventional single intermediate electrode of the electrostatic lens 16, with the electrode portions at the ends of the array being at a different potential than the end electrodes of equal potential and with at least one intermediate electrode portion being at a potential different from the end portions and approaching the potential of the end electrodes to reduce the angles of the lines p (FIG. 3C) of equal potential with respect to the tube axis xx and to decrease the potential gradient along such axis, as indicated at P on FIG. 3A, whereby to make the electrostatic lens equivalent to an optical lens L (FIG. 3A) of relatively large diameter and having surfaces of large radii of curvature.

As shown on FIG. 2, in which the several parts ofan electrostatic focusing lens 16a are identified by the same reference numerals employed in connection with the above description of FIG. I, but with the letter a appended thereto, the electrostatic focusing lens 16a provided according to this invention is there shown to be comprised of end electrodes 17a and 18a, and an intermediate electrode assembly, generally indicated by the reference numeral 19a, in the form of an axial array of three electrode portions 30, 31, and 32. Although assembly 19a is shown for purposes of illustration to have three electrode portions, a larger, preferably odd number of electrode portions may be provided.

In the illustrated embodiment, the end electrode portions 30, 32 of the axial array constituting electrode are at a potential substantially different from the potential of the end electrodes 17a and 18a, and may be connected to each other by a conductor 33. More specifically, the potential of electrode portions 30 and 32 may be substantially lower than the potential of end electrodes 17a and 18a, and may typically be 0 v. to 600 v. compared'to l3 kv. to 20 kv. for end electrodes 17a and 18a. The potential difference provides an electric focusing field between the end electrodes 17a, 18a and the intermediate electrode assembly 19a. A

The intermediate electrode portion 31 of the axial array is at a potential which deviates or is different from the potential of end electrode portions 30, 32 in the direction toward the potential of end electrodes 17a and 18a, and may typically be at the same relatively high potential, such as the anode voltage applied to the conductive layer at the inner surface of the tube, as is applied to end electrodes 17a and 18a. The means for applying this deviating potential to intermediate electrode portion 31 may be a separate source of potential (not shown) or, when it is desired to apply the same potential to intermediate electrode portion 31 as is applied to end electrodes 17a and 18a, electrode portion 31 may be connected to the end electrodes 17a and 18a by a conductor 35 connected to the end electrode interconnection 20a.

The overall effect of the intermediate electrode assembly 19a, with the potential differences associated with its various electrode portions 30, 31, 32, is to very substantially reduce the potential gradient along the tube axis between electrodes 17a and 18a and to decrease the angles with respect to the tube axis of the lines of equal potential within the field, with the result that the equivalent optical lens L (FIG. 3A) is of large diameter and has surfaces of large radii of curvature, as is desired.

The electrode portions 30, 31, 32 comprising intermediate electrode assembly 19a are shown spaced apart from each other in the axial array and, when assembly 19a has an odd number of at least three electrode portions, as is preferred, at least the intermediate electrode portion situated at the middle of the axial array is at a higher potential than the end electrode portions.

Of course, the surface radii of the equivalent optical lens can also be changed by changing the distance between electrodes 17a and 18a, the potential difference between electrodes 17a and 18a and end'electrode portions 30 and 32, and the potential difference between end electrode portions 30 and 32, and the intermediate electrode portion 31. Thus, the invention permits an electrostatic focusing lens to be obtained that is equivalent to an optical lens with precisely desired surface radii.

Although the invention has been described above with reference to its application to single-beam cathode ray tubes, reference to FIG. 4 will show the application of the invention to a single-gun, plural-beam cathode ray tube of the type disclosed in detail in U.S. Pat. No. 3,448,316. In the cathode ray tube of FIG. 4, three electrically separated cathodes K K and K, have red, green" and blue video signals respectively supplied thereto. The three cathodes are arranged with their electron emitting surfaces in a straight line so as to be aligned with similarly arranged apertures in a first grid (3,. A second cup-shaped grid G has an end plate disposed adjacent grid G, and formed with apertures aligned with the apertures of first grid 6,. Arranged in order following the grid G in the direction away from control grid G, are an open-ended tubular electrode 117, an electrode assembly 119 consisting of an axially array electrode portions 130, 131, 132, and an openended tubular electrode 118 constituting an electrostatic focusing lens 116. Electrode 117 includes a relatively small diameter end portion 117a, and is supported with such end portion extending into cup-shaped grid G and spaced radially from the side wall of the latter.

When voltages similar to those indicated for the cathode ray tube of Fit]. 1 are applied to grids G and G, and electrodes 117, 118, and to the portions of electrode assembly 119 in a manner in accordance with the invention, beams B 8 and B emitted by cathodes K K and K are modulated with the three different video signals applied between grid G, and the respective cathodes. Grid G and the end portion 117a of electrode 117 cooperate to provide an electric field defining an electrostatic beam converging lens illustrated in broken lines by its optical equivalent 1 and which is operative to converge beams B and B toward beam B so that the three beams cross each other substantially at the location of the optical center of the focusing lens 116.

in order to cause convergence of the beams B and B which emerge from electrode 118 along divergent paths, the electron gun of FIG. 4 further has deflecting means 36 that includes shielding plates 37 and 37 provided in spaced opposing relationship to each other and extending axially away from the free end of electrode 118. Deflecting means 36 further includes converging deflector plates 38 and 38', which may be flat, as shown, or outwardly convexly bent or curved, and which are mounted in spaced opposing relation to the outer surfaces of shielding plates 37 and 37' respectively. The plates 37 and 37' and the plates 38 and 38' are disposed so that the beams B B and B pass between the plates 37 and 38, between the plates 37 and 37' and between the plates 37 and 38, respectively. The outer plates 38 and 38 may be mounted by attachment to electrode 118, as shown, while plates 37 and 37 are supported from plates 38 and 38' and insulated therefrom, as by insulating supports 39.

A high anode voltage V,,, for example of 13 to 20 kv., provided by a source 40 is applied by way of an anode button 41 to the usual conductive layer 42 lining the tube envelope, and a spring contact 43 extends from plate 37 into engagement with layer 42. The high voltage V thus applied to plate 37 is transmitted to plate 37 by a conductor 44 therebetween. A voltage (V,,V,) which is lower than the voltage V, by 200 to 300 v., constituting a convergence voltage, is applied to outer plates 38 and 38'. The source of the convergence voltage V, is indicated at 45 and may provide a static convergence voltage and also, if desired, a dynamic convergence voltage varied in accordance with the scanning action. As shown, the voltage (V -V may be applied by way of a button 46 in the tube neck 47 and a conductor 48 to electrode 117 of focusing lens 116. Further, electrode 118 and intermediate electrode portion 131 are connected with electrode 117 by conductors 49, to receive t ee (VF V0 aadts zise x lerp a ss 38 and 38 are mounted directly on electrode 118, plates 38 and 38' also receive the voltage (V,,V,). Thus, convergence voltage differences V are applied between plates 37 and 38 and between plates 37' and 38' so that beam B and 8,, will cross each other and beam 8 at a common spot on an aperture grill or mesh 51 and diverge therefrom to strike respective color phosphors b, r and g arranged in suitable arrays to constitute the color screen 52 on the face plate 53 of the tube. A deflection yoke 54 is also provided to cause beams B B and 13,, to simultaneously scan screen 52 in the usual manner. The end electrode portions 130, 132 may be electrically connected with a source at low potential, as previously described with respect to similar electrode portions 30, 32 of FIG. 2, by a conductor 55 and may be electrically connected together by another conductor 56 so as to have the same potential applied to both portions 30, 32. The conductor 55 may be electrically connected to one of the pins of the tube, which in turn may be connected to the source of low potential (not shown). Since beams B B and B all pass substantially through the optical center of electrostatic focusing lens 116 so as to be focused thereby on screen 52, lens 116 imparts diminished aberration to the resulting beam spots on the screen as compared with earlier arrangements in which, for example, beams 13,; and B pass through the focusing lens at substantial distances from its optical axis. However, since beams 13,, and 13,, pass through lens 116 at substantial angles to the optical or tube axis, op-

timum reduction or avoidance of aberrations of the beam spots requires that lens 116 be equivalent to a large diameter optical lens having surfaces with large radii of curvature. Thus, in accordance with this invention, the optical lens L equivalent to electrostatic focusing lens 116 is made to have a large diameter and surfaces with large radii of curvature by providing lens 116 with the intermediate electrode assembly 119 comprised of an axial array of at least three annular electrode portions 130, 131, 132, having the potential differences associated therewith, as previously described. Since the middle electrode portion 131 of the array is at a different potential than end electrode portions 130, 132, the effect is to reduce the potential gradient of the electric field of lens 116, and thus to provide the latter with the desired Optical equivalent of a lens of large diameter and large radii of curvature.

Although illustrative embodiments of electrostatic lenses according to this invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments and that various changes and modifications may be made therein by one skilled in the art without departing from the scope or spirit of the invention.

What is claimed is:

1. In a cathode ray tube having beam producing means generating at least one electron beam and a phosphor screen positioned to have the beam impinge thereon; electron focusing lens means for focusing the beam on the screen comprismg:

first and second annular electrodes coaxial with the longitudinal axis of the tube and being axially spaced from each other,

an annular electrode assembly coaxial with said axis and extending between and partly over said first and second electrodes,

said annular electrode assembly including an axial array of at least three annular electrode portions, means for applying to the electrode portions at the ends of said axial array a potential substantially different from the potential of said first and second electrodes to provide an electric focusing field between said first and second electrodes and said electrode assembly,

and means applying to at least one of said electrode portions intermediate said end electrode portions a potential that deviates from said potential of said end electrode portions in the direction toward said potential of the first and second electrodes whereby to reduce the potential gradient of said field.

2. A cathode ray tube according to claim 1, in which:

said deviating potential applying means applies to said one electrode portion the same potential as that of said first and second electrodes.

3. A cathode ray tube according to claim 1, in which:

said electrode portions are spaced from each other.

4. A cathode ray tube according to claim 1, in which:

there are an odd number of said electrode portions in said array, and said one electrode portion is at the middle of said axial array.

5. A cathode ray tube according to claim 1, in which:

said beam producing means generates a plurality of electron beams impinging on said screen and which are made to intersect each other at a location in the tube between said beam producing means and said screen, and in which said electron focusing lens means has an optical center and is disposed to position said optical center substantially at said location where the beams intersect each other.

6. A cathode ray tube according to claim 1, in which:

said end electrode portions are at a potential substantially lower than said first and second electrodes.

7. A cathode ray tube according to claim 6, in which:

said deviating potential applying means applies to said one electrode portion the same potential as that of said first and second electrodes. 

1. In a cathode ray tube having beam producing means generating at least one electron beam and a phosphor screen positioned to have the beam impinge thereon; electron focusing lens means for focusing the beam on the screen comprising: first and second annular electrodes coaxial with the longitudinal axis of the tube and being axially spaced from each other, an annular electrode assembly coaxial with said axis and extending between and partly over said first and second electrodes, said annular electrode assembly including an axial array of at least three annular electrode portions, means for applying to the electrode portions at the ends of said axial array a potential substantially different from the potential of said first and second electrodes to provide an electric focusing field between said first and second electrodes and said electrode assembly, and means applying to at least one of said electrode portions intermediate said end electrode portions a potential that deviates from said potential of said end electrode portions in the direction toward said potential of the first and second electrodes whereby to reduce the potential gradient of said field.
 2. A cathode ray tube according to claim 1, in which: said deviating potential applying means applies to said one electrode portion the same potential as that of said first and second electrodes.
 3. A cathode ray tube according to claim 1, in which: said electrode portions are spaced from each other.
 4. A cathode ray tube according to claim 1, in which: there are an odd number of said electrode portions in said array, and said one electrode portion is at the middle of said axial array.
 5. A cathode ray tube according to claim 1, in which: said beam producing means generates a plurality of electron beams impinging on said screen and which are made to intersect each other at a location in the tube between said beam producing means and said screen, and in which said electron focusing lens means has an optical center and is disposed to position said optical center substantially at said location where the beams intersect each other.
 6. A cathode ray tube according to claim 1, in which: said end electrode portions are at a potential substantially lower than said first and second electrodes.
 7. A cathode ray tube according to claim 6, in which: said deviating potential applying means applies to said one electrode portion the same Potential as that of said first and second electrodes. 